1. Field of the Invention
The present invention relates to the calibration of epitaxy reactors, and more specifically, to reactors likely to be used in the solid-state component industry.
2. Discussion of the Related Art
Solid-state components are being manufactured on wafers having larger and larger diameters. For example, wafers having diameters of 20 centimeters are used industrially, and there are ongoing projects to develop machines adapted to process wafers having still larger diameters. With wafers having such large diameters, processing of the wafers becomes difficult, especially processings involving vapor phase depositions, for example, depositions of epitaxied layers.
Since it is generally desirable to obtain a great number of identical components on the same layer, it is of major importance that the depositions performed have the same characteristics across the entire wafer. For this reason, the temperature at which the wafer is brought must notably correspond to a selected temperature and be uniform across the whole wafer.
FIGS. 1A and 1B very schematically show a side view and top view, respectively, of an example of an epitaxy reactor. This reactor includes a quartz chamber 1 with a cross-section generally shaped as a rectangle, with its height b being small with respect to its width l. For example, if it is desired to process wafers 10 with a diameter of 20 cm, the width clearly must be greater than 20 cm while the height need only be of a few centimeters. The top view of the chamber also has a generally rectangular shape. Gases are introduced at an injection flange 2 having an inlet nozzle 3 and are drained off at an exhaust flange 4 having an exhaust nozzle 5. Closing plates 7 and 8 can be opened to access the inside of the tube, for example, to introduce and retrieve samples to be processed. These various elements are mounted together under pressure, using O-rings 9 therebetween. The sample 10 to be processed, for example, a silicon wafer, is laid on a tray or susceptor 11. This tray generally is a rotating tray that is rotated during the processing to enhance the uniformity of the deposition.
A system for heating, for example, heating lamps 13 and 14, is arranged so as to expose to radiation the upper surface of the sample and the lower surface of the tray on which it is laid. The lamp sets 13 and 14 each are divided into several lamp subsets, for example, ten subsets. Each of the lamp subsets is adjustable independently, in order to be able, by an appropriate setting, to obtain a uniform temperature throughout wafer 10.
Of course, FIGS. 1A and 1B are extremely simplified and an actual reactor will be more complex than the system shown. Closing plates 7 and 8 preferably will be associated with robotized inlet chambers. Complex gas supplying systems also generally will be provided, and an accurate mass flowmeter will be connected in series between nozzle 3 and the source of the reaction gas(es). To ensure that the gas circulation within the chamber is uniform, several injection slots 15 extend widthwise across the chamber.
The reactor shown and described hereinabove is shown and described only as an example. Chambers made of quartz or other materials or of different shapes, for example, with a cylindrical symmetry, also may be used.
A problem that arises with such epitaxy reactors is the inability to obtain a proper temperature setting of the lamps or other means for heating silicon wafer 10.
A ring 18 often is provided around rotating tray 11, having its inner edge extremely close to the circumference of the silicon wafer and its upper surface substantially in the same plane as the silicon wafer. This ring typically supports several temperature sensors, for example, three thermocouples TF, TS and TR, which are located, respectively, upstream of the wafer (i.e. on the surface first receiving the gas flow), on one surface, and downstream of the wafer with respect to the gas flow. Moreover, a central thermocouple (not shown) generally is installed in susceptor 11. However, these thermocouples do not give an exact image of the wafer temperature. Rather, they are brought to temperatures that are different from those of the wafer. These temperatures therefore may be used only as information for correcting and regulating the temperature. Also known in the art are epitaxy reactors with temperature sensors arranged differently and/or of different types, for example, optical pyrometers. Systems for differentially controlling the power injected into the different heating areas also are used.
Conventionally, in the different existing devices for processing silicon wafers, to perform an initial calibration of a parameter such as temperature, as well as a periodical setting of the parameter, a reference wafer is placed in the device and is submitted to a specific processing (which is one of the processings usually performed by the device), and the possible setting defects of the parameter considered are inferred from the structure resulting from the processing. For example, in a device for chemical vapor deposition enabling the performance of silicon oxide depositions, the thickness of the oxide layer is analyzed in different locations on the wafer and the fluctuations of the parameter considered are inferred from the variations in this thickness. Similarly, in a rapid thermal annealing (RTA) device, which usually is used to perform activation annealings on implanted layers, a reference wafer having undergone an implant in the device is placed therein and is submitted to a high temperature rapid annealing, after which, the resistance of the activated layer is measured at different locations on the layer. The existence of thermal variations is inferred from the variations in this sheet resistance.
However, in a device for performing epitaxial depositions, that is, in devices wherein the working environment is non-oxidizing, if a thin layer is deposited epitaxially, for example, in a polysilicon deposition, the potential inhomogeneity (i.e., a variation in thickness and/or doping) of the deposition will depend only partially on the temperature, and also will depend on many other parameters, such as the composition of the deposition gas and the distribution of the gas flow across the surface of the layer. It therefore is not possible to isolate the action of the temperature parameter by these means.
In practice, it has been found that if the temperature of a silicon wafer during an epitaxial deposition is unequal while operating at high temperatures, the existence of inhomogeneities will cause crystalline defects, commonly called striplines, to appear in the wafer.
Accordingly, a method currently used to test the uniformity of the temperature in an epitaxy reactor consists of inserting a reference silicon wafer therein, submitting the wafer to a thermal cycle up to a desired temperature (for example 1050.degree. C.), and removing the wafer from the reactor and analyzing the crystalline defects. The disadvantage of this method is that, even though the occurrence of crystalline defects reveals the existence of thermal inhomogeneities, there is no simple correlation between these thermal inhomogeneities and the crystalline defects. For example, when a strip-line is seen to appear, it can only be inferred therefrom that the temperature of the layer on both sides of this line is unequal and that the setting of the lamps needs to be modified. But, it is not known whether this modification should entail an increase or a decrease in temperature. This technique thus requires to a long trial and error process before a setting enabling the suppression of the striplines can be found. Even when this result has been achieved, however, the setting remains non-ideal. Indeed, the crystalline defects may seem to disappear as soon as the setting merely approximates the optimal setting, i.e., without actually being optimal. Further, certain types of thermal inhomogeneities do not generate crystalline defects. For example, linear temperature variations will not be detectable.
The other known methods of obtaining a uniform temperature setting also are not very satisfactory. They essentially consist of performing tests with an increased number of thermocouples attached to a reference wafer 10 and to the tray 11 receiving the wafer. For, even if these methods can obtain an accurate initial setting, they are poorly adapted to periodical resettings and are not compatible with the rotating of the wafer.